1. Technical Field
The present invention relates to signal modulation and more particularly to a modulation system and method that improve power efficiency and reduces inter-carrier interference (ICI).
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) has achieved great success in wireless and cable communications due to its robustness against multipath fading and its potential for high spectral efficiency. However, OFDM is sensitive to frequency distortion. For optical fiber communication, chromatic dispersion (CD) and polarization mode dispersion (PMD) have similar frequency distortion effects. To compensate for these dispersions, and to take advantage of the high spectral efficiency, the OFDM applications in optical systems have begun to be investigated.
Directly applying OFDM into optical systems has at least the following problems: (a) sensitivity to carrier frequency offset (CFO) caused by the misalignment in carrier frequencies between transmitter and receiver; (b) sensitivity to the receiver In-phase and Quadrature (IQ) imbalance due to manufacturing inaccuracies. Both CFO and IQ imbalance will cause Inter-Carrier Interference (ICI) among the sub-carriers and decrease the Carrier-to-Interference Ratio (CIR), thus degrading system performance.
Referring to FIG. 1, a signal constellation for 8-PSK (8-phase shift keying) is illustratively shown. OFDM systems using conventional 8-PSK, whose signal constellation is shown in FIG. 1, use all sub-carriers simultaneously to transmit modulated symbols. The number of sub-carriers and the separation between the sub-carriers are fixed all the time. When the CFO and the IQ imbalances exist, all sub-carriers will contribute to the ICI, which will degrade system performance. Due to the fact that all sub-carriers are used all the time, there is no power efficiency improvement.
An OFDM system using 8-PSK modulation uses modulated signal waveforms for 8-PSK which can be expressed as
                                                                        s                m                            ⁡                              (                t                )                                      =                                                            g                  ⁡                                      (                    t                    )                                                  ⁢                                  cos                  ⁡                                      [                                                                  2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          c                                                ⁢                        t                                            +                                                                                                    2                            ⁢                            π                                                    8                                                ⁢                                                  (                                                      m                            -                            1                                                    )                                                                                      ]                                                  ⁢                                                                  ⁢                1                            ≤              m              ≤              8                                ,                      0            ≤            t            ≤            T                          ⁢                                  ⁢        where                            (        1        )            g(t) is the pulse shape, t is time, T is the period or symbol duration, fc is the carrier frequency.